Linux

Linux is a family of open source operating systems stemming from Unix. Common Linux distributions include: Debian, Ubuntu, and Fedora. Personally, my Linux distribution of choice is Arch Linux.

Command Line Interface

All Linux distributions have a command line interface (CLI) usually running bash. It is a way to quickly and efficiently perform tasks on the computer using available commands. The CLI is particularly suited for automation of repetitive or delayed tasks, and provides very simple inter-process communication.

I usually do 90% of my work on the command line because I have found it to be faster and easier to perform tasks (especially if it is repetitive). My shell emulator of choice is zsh for its many plugins.

File System

In Linux, everything is a file. This means that all connected hardware is represented as a file. Files are files. Directories are files. Sockets are files. Everything. This concept means the same tools can be applied when manipulating or interacting with devices.

Kernel and User Space

System memory is divided into two distinct spaces: kernel space and user space. Kernel space is the protected memory space that runs the operating system. It has full access to the hardware, contains kernel code, and most of the memory is directly mapped to physical memory.

User space is where the normal application runs. It has limited access to resources and contains process code, data, and memory mapped files. User programs can request resources through system calls to the kernel.

Cron

Cron is a time based job scheduler. Each line in a cron table file represents a job. Here is a template describing each entry in the table.

# ┌───────────── minute (0 - 59)
# │ ┌───────────── hour (0 - 23)
# │ │ ┌───────────── day of the month (1 - 31)
# │ │ │ ┌───────────── month (1 - 12)
# │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday;
# │ │ │ │ │                                   7 is also Sunday on some systems)
# │ │ │ │ │
# │ │ │ │ │
# * * * * * command to execute

Anacron

Anacron is similar to cron, except it does not expect the system to always be continuously running. In other words, anacron will reschedule jobs if the system is off at the time the job was supposed to run, unlike cron which will skip the job entirely.

Tools

Working in Linux and the CLI, you will invariably learn many tools for system administration and development. Below is a list of common commands in no particular order.